Microprocessor-controlled multifunctioning light with intrinsically safe energy limiting

Abstract

A method is provided for reducing the power rating of a current limiting resistor (R) in a branch circuit having at least one protected element and the current limiting resistor connected between first and second nodes. The method includes the steps of: determining a maximum fault current in the branch circuit; determining a total current limiting resistance to limit the current in the branch circuit having the short-circuited elements to the maximum fault current; inserting a fuse having an intrinsic resistance in the branch circuit; and dividing the determined total current limiting resistance between the resistor (R) and the intrinsic resistance of the fuse.

1. A method of reducing the power rating of a current limiting resistor (R) in a branch circuit having at least one protected circuit element and the current limiting resistor connected between first and second nodes, comprising the steps of:
 determining a maximum fault current in the branch circuit with each protected circuit element in the branch short-circuited; 
 determining a total current limiting resistance to limit the current in the branch circuit having the short-circuited protected circuit elements to the maximum fault current; 
 inserting a fuse in the branch circuit selected to have an intrinsic resistance and a current rating to open at a current exceeding the maximum fault current in the branch circuit; and 
 dividing the determined total current limiting resistance between the current limiting resistor (R) and the intrinsic resistance of the fuse. 
 
     
     
       2. The method of  claim 1 , wherein the step of determining a total current limiting resistance in the branch circuit comprises a step of:
 dividing a maximum available voltage across the branch circuit by a maximum fault current in the branch circuit. 
 
     
     
       3. The method of  claim 1 , wherein the sum of the respective resistances of the current limiting resistor (R) and the fuse equals the determined total current limiting resistance. 
     
     
       4. The method of  claim 1 , wherein the step of inserting comprises the steps of:
 selecting the fuse based on its current rating multiplied by a derating factor; 
 determining the intrinsic resistance of the selected fuse; and 
 subtracting the intrinsic resistance from the total determined resistance to obtain the resistance value of the current limiting resistor (R). 
 
     
     
       5. The method of  claim 4 , wherein a step of inserting further comprises a step of:
 connecting the fuse having the intrinsic resistance and the current limiting resistor (R) in the branch circuit in series with the protected elements. 
 
     
     
       6. The method of  claim 1 , further comprising a step of:
 determining the power dissipation required of the current limiting resistor (R). 
 
     
     
       7. The method of  claim 6 , further comprising the steps of:
 selecting the fuse based on it current rating multiplied by a de-rating factor; determining the intrinsic resistance of the selected fuse; and subtracting the intrinsic resistance from the total determined resistance to obtain the resistance value of the current limiting resistor (R). 
 
     
     
       8. The method of  claim 1 , wherein the step of dividing comprises a step of:
 providing substantially more of the total current limiting resistance in the intrinsic resistance of the fuse than in the current limiting resistor R such that the power dissipation rating of the current limiting resistor (R) is reduced. 
 
     
     
       9. The method of  claim 1 , wherein the step of dividing comprises a step of:
 permitting a substantial reduction in the physical size of the current limiting resistor (R) to fit within an available space in a designated apparatus. 
 
     
     
       10. The method of  claim 1 , wherein an intrinsic resistance portion of the total current limiting resistance is supplied by two fuses connected in series, wherein the rating of one designated fuse provides a circuit protection capability in the branch circuit.